What are anticipated use cases for Is Diffuse Ray (Light Path Node) in Eevee?
While with regular viewport rendering alone it is hard to find a use case, while using probes it can have a visible effect.
If you use an Irradiance Volume probe and bake baking indirect lighting, you use it to change object color for direct visibility, while indirect light sees it differently.
In the example below the sphere color is is red, but through the Is Diffuse output we tell the rays bouncing off the surface it is blue, as such you can see a faint bluish tint to the color bleed effect on the wall.